I'm saying you should make assemblies with many different k-mer values and then in the end you merge all the assemblies into one. Also, you obviously can't have larger k-mer values than your read length, I mean, how would you e.g. get a 120-mer from a 90 bp read?

This is precisely the opposite, it is designed to choose k for you. Just run kmergenie without any parameter, and it will explicitly print what it thinks is a good kmer size. Most of the time, the value is accurate.

Additionally, for unusual cases, you can take a look at the generated HTML report to make a more informed decision.